#include <bits/stdc++.h>
using namespace std;
#define int long long
const int INF = 1e18;
const int MOD = 1e9+7;
const int MAXN = 5e5;
int n, m;
vector<int> adj[MAXN+5];
int inDegree[MAXN+5];
int visited[MAXN+5];
int dp[5][MAXN+5];
vector<pair<int, int>> pre[5][MAXN+5];
int entered[MAXN+5];
set<int> ans;
void backtrack(pair<int, bool> yey){
auto [cur, stat] = yey;
// cerr << "backtrack : " << cur << ' ' << stat << endl;
if(stat) ans.insert(cur);
for(auto i : pre[stat][cur]) backtrack(i);
}
void debug(){
cerr << "\nADJ" << endl;
for(int i = 1; i <= n; i++){
cerr << i << " : ";
for(auto j : adj[i]) cerr << j << ' ';
cerr << endl;
}
cerr << "\nDP" << endl;
for(int i = 1; i <= n; i++){
cerr << i << " : " << dp[0][i] << ' ' << dp[1][i] << endl;
}
cerr << "\nPRE" << endl;
for(int i = 1; i <= n; i++){
cerr << i << ' ' << 0 << " : ";
for(auto j : pre[0][i]) cerr << j.first << ' ' << j.second << ' ';
cerr << endl;
cerr << i << ' ' << 1 << " : ";
for(auto j : pre[1][i]) cerr << j.first << ' ' << j.second << ' ';
cerr << endl;
}
}
void solve(){
cin >> n >> m;
for(int i = 1; i <= m; i++){
int u, v; cin >> u >> v;
adj[v].push_back(u);
inDegree[u]++;
}
queue<int> bfs;
for(int i = 1; i <= n; i++){
if(inDegree[i] == 0) bfs.push(i);
}
vector<int> lastNode;
while(!bfs.empty()){
int cur = bfs.front();
bfs.pop();
// cerr << "cur : " << cur << endl;
visited[cur] = 1;
dp[1][cur]++;
bool isLastNode = 1;
for(auto next : adj[cur]){
if(visited[next]) continue;
isLastNode = 0;
// cerr << "next : " << next << endl;
if(dp[0][cur] > dp[1][cur]){
dp[0][next] += dp[0][cur];
pre[0][next].push_back({cur, 0});
}else{
dp[0][next] += dp[1][cur];
pre[0][next].push_back({cur, 1});
}
dp[1][next] += dp[0][cur];
pre[1][next].push_back({cur, 0});
entered[next]++;
if(entered[next] >= inDegree[next]) bfs.push(next);
}
if(isLastNode) lastNode.push_back(cur);
}
for(auto i : lastNode){
// cerr << "lastNode : " << i << endl;
if(dp[0][i] > dp[1][i]) backtrack({i, 0});
else backtrack({i, 1});
}
cout << ans.size() << endl;
for(auto i : ans) cout << i << ' ';
cout << endl;
// debug();
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int tc = 1;
// cin >> tc;
while(tc--){
solve();
}
return 0;
}
/*
4 3
1 2
1 3
2 4
=> 2 {4, 1}
4 4
1 2
1 3
2 4
3 4
=> 2 {4, 1}
*/
